How can I clear the weight from outsiders vertex when weight painting?
Let's say we have a nice and happy cube



Oh no they got sick!



The doctor said that this cube needs to lose some weight and eat healthy paint :D

One way the cube decided to lose weight was to select all the vertices, it set the strenght to 0 and presses Shift + K  to apply the strenght to all the vertices



The little cube was surprised how quickly this method was, and so he wondered "is there another method" so he browsed on Google about health diets and found this other method with same results:

Select all the vertices you want to clear their weight, now select the vertex group of the bone that you desire to remove, now click on Remove and it will remove all weight from that bone



The little cube was very happy and decided to live a healthy life, found love, got married, and had children